a hydrophilic binder comprising a mixture of polyvinyl alcohol and a vinylpyrrolidone homopolymer and/or vinylpyrrolidone copolymer, and

a water soluble compound containing aldehyde groups.

2. The material of claim 1, wherein said water soluble compound containing aldehyde groups is selected from the group consisting of a melamine formaldehyde condensation product and a polyvalent aldehyde.

3. The material of claim 2, wherein the ratio of the water soluble compound containing the aldehyde groups to the hydrophilic binder is about 1:1 to 1:40.

4. The material of claim 3, wherein said ratio is about 1:1.5 to 1:27.

5. The material of claim 1, wherein the ratio of the water soluble compound containing the aldehyde groups to the hydrophilic binder is about 1:1 to 1:40.

6. The material of claim 5, wherein said ratio is about 1:1.5 to 1:27.

7. The material of claim 1, wherein the amount of water soluble compound containing aldehyde groups in the ink receiving layer is between about 2 to 15 wt %.

8. The material of claim 7, wherein said amount is between about 3 to 10 wt %.

a hydrophilic binder comprising a mixture of polyvinyl alcohol and a vinylpyrrolidone homopolymer and/or vinylpyrrolidone copolymer,

a water soluble compound containing aldehyde groups, and

a quaternary ammonium compound.

Description

This is a continuation of application Ser. No. 08/261,155, filed Jun. 17, 1994 now abandoned.

BACKGROUND, SUMMARY AND DESCRIPTION OF THE INVENTION

This invention relates to a printing material for the ink jet printing process and a coating compound for preparing an ink receiving layer for the material.

In ink jet printing systems printing operations are carried out by forming droplets of ink by means of various ink jet methods, such as electrostatic attraction methods, bubble formation processes, etc., and these droplets of ink are applied to a printing material. Such printing processes make it possible to print images with a very high resolution directly from electronic data. The image receiving materials used for this purpose must meet certain high requirements so that the image produced by the ink jet process will:

have a high resolution;

have a high color density;

have sufficient color gradations;

be smudgeproof; and

be waterfast.

In order to satisfy these requirements or goals, the following basic conditions must be met:

the ink must be rapidly absorbed by the printing material;

the droplets of ink sprayed on the printing material must spread out in the most accurate possible manner in the shape of a circle and with precisely defined outlines;

the ink diffusion in the printing material must not be too great so that the diameter of the ink spots does not increase any more than is absolutely necessary;

when one ink spot overlaps with another ink spot which was previously applied, it should not have any negative effect or smear;

the printing material must have a surface that permits a high visual reflection density and a high brilliance of the dyes; and

the printing material should have a high dimensional stability and should not stretch after the printing process.

Some of these requirements are contradictory to each other. For example, if the material becomes smudgeproof too rapidly, there will be little or no spreading of the droplet of ink and, thus, the clarity of the resulting image is impaired. On the basis of the requirements of the printing material, there has been a search for ways to obtain an image with the highest possible ink density, while still being as smudgeproof as possible.

Papers in which the ink fluids can be absorbed in the spaces formed between the pulp fibers in the paper or between the fibers and filler have been used as the printing material for ink jet printing. Another group of printing materials include papers having a special ink receiving layer.

The ink receiving layers consist essentially of a pigment/binder mixture. In addition to increasing the whiteness of the printing material, the pigments serve the function of retention of the dyestuffs from the printing ink on the surface of the sheet. A high pigment concentration leads to a high porosity of the layer (German Patent No. 30 24 205). This makes the paper quite smudgeproof. However, at the same time the dyes are also drawn out of the ink into the interior of the printing material, and this has a negative effect on the color density of the image.

Japanese Patent JP 61-041585 discloses a method of producing printing material with a receiving layer of polyvinyl alcohol and polyvinylpyrrolidone. The mixing ratio of the two components PVA to PVP is 3:1 to 1:5. However, a disadvantage of this material is its inadequate waterfastness and its wet rub off properties.

Japanese Patent JP 61-261089 discloses a transparent material for overhead projectors which contains a cationic conductive resin in addition to a mixture of polyvinyl alcohol and polyvinylpyrrolidone. This makes the paper smudgeproof and waterproof, but the wet rub off properties are not adequate.

Therefore, the object of the present invention is to provide a printing material for the ink jet printing process which will fulfill the requirements mentioned above, but specifically will assure a good waterfastness and wet rub off properties, in addition to a high color density and image definition or clarity.

This object is achieved by means of a printing material comprising a support and an ink receiving layer applied to the support and containing a polyvinyl alcohol, polyvinylpyrrolidone homopolymer and/or vinylpyrrolidone copolymer, and a water soluble substance containing aldehyde groups.

The water soluble substance containing aldehyde groups may be a melamine formaldehyde condensation product containing free CHO groups in the amount of at least 0.05 mol%. However, a polyvalent aldehyde, such as an oxalaldehyde, may also be used.

The quantity ratio of the water soluble substance to the hydrophilic binder present in the ink receiving layer which is a mixture of polyvinyl alcohol, polyvinylpyrrolidone homopolymer and/or polyvinylpyrrolidone copolymer, is 1:1 to 1:40. In a preferred embodiment, the ratio is 1:1.5 to 1:27.

The quantity ratio of polyvinyl alcohol to polyvinylpyrrolidone homopolymer and/or copolymer in the binder mixture is 10:1 to 4:1.

In a special embodiment of the invention, the polyvinylpyrrolidone copolymer is a vinylpyrrolidone vinyl acetate or a vinylpyrrolidone styrene copolymer.

The amount of water soluble substance containing aldehyde groups in the ink receiving layer is 2 to 15 wt %, especially 3 to 10 wt %.

The amount of binder in the layer is 15 to 80 wt %, especially 30 to 60 wt %.

In a special embodiment of the invention, the ink receiving layer contains a quaternary ammonium compound with a certain cationicity which is determined with PCD titration with a 1×10-3 n solution of sodium polyethylenesulfonate (PES solution). The values for the cationicity obtained by this method for the ammonium compounds according to this invention amount to 15 to 30 ml PES solution, especially 20 to 25 ml PES solution. The quaternary ammonium compounds include, for example, polyamine salts and polyamideamine compounds. Polydiallyldimethylammonium chloride has proven to be especially advantageous.

The ink receiving layer according to the invention may also contain other additives, such as white pigments, colored pigments, dyes, dispersants, wetting agents, curing agents and optical brighteners.

Pigments that can be used in the ink receiving layer include silicic acid, clay, zeolites and other inorganic pigments. In a preferred embodiment of this invention, an amorphous silicic acid having a pore volume of 1.0 to 2.5 ml/g with a particle size of ≦5 μm is contained in the ink receiving layer. The amount of pigment in the ink receiving layer is 10 to 80 wt %, especially 40 to 65 wt %.

The ink receiving layer is applied to the carrier from an aqueous dispersion with the help of any of the conventional methods of application and metered addition. The coating weight of the ink receiving layer is 0.5 to 15 g/m2, preferably 2 to 8 g/m2. A plastic film or a coated or uncoated base paper may be used as the support. The base paper may be paper sized with an acidic or neutral size. A base paper sized with diketene and containing 5 to 20 wt % pigment and/or filler, such as TiO2, CaCO3 and SIO2, is preferred.

In another preferred embodiment of the invention, the back side of the base paper has a layer containing a hydrophilic colloidal binder such as starch, modified starch, polyvinyl alcohol or gelatin. An oxidized potato starch is especially preferred.

The layer on the back side may contain up to 60 wt % fillers, such as for example, clay, zeolite, CaCO3, and pigments and other additives.

The invention will be illustrated in greater detail in the following examples.

EXAMPLE 1

The front side of raw paper with a basis weight of 80 g/m2 containing 20 wt % CaCO3 in the pulp, and sized with a neutral alkylketene dimer size was coated with an aqueous coating compound and then dried. The resulting ink receiving layers had the following composition:

The printing material obtained in the comparative examples was printed in a thermal jet process and then analyzed. The results are summarized in Table 3.

In addition to the comparative examples, two commercial printing materials were also used and analyzed. The test results are also summarized in Table 3.

Testing the Printing Material Obtained According to the Examples and Comparative Examples

The printing material was printed with a Hewlett Packard HP Deskjet 550 C that operates according to the bubble jet principle (thermal jet).

The color density, definition, waterfastness and wet rub off properties were tested on the resulting print images.

The density measurements were performed with an Original Reflection Densitometer SOS-45. The measurements were performed for the primary colors cyan, magenta, yellow and black.

For determining the waterfastness of the paper, the printing material was immersed in water. The density (%) remaining after 60 seconds in the water bath is used as a measure of the water stability.

The image clarity (definition) is determined with a fiber counter. Field distances between a red field and a green field or between two black fields are determined (maximum value 1 mm). The colored fields composed of the primary colors have 200% ink coverage. Therefore, they serve as test fields for fixing large quantities of ink.

To determine the wet rub off properties, a 1 kg weight covered with a wet towel is passed five times over a test strip printed with 100% of black or cyan ink, and the density loss is evaluated (grades of 1 to 5, wherein grade 1 is very good and grade 5 is poor).

by keeping the percent by volume of inorganic particles in a specified range, lower than 65% on the image receiving layer that also contains a polymeric binder and crosslinking agent, and using an ink of given viscosity and surface tension